Download Sex-linked Inheritance

Sex-linked Inheritance
Jessica Harwood
Douglas Wilkin, Ph.D.
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AUTHORS
Jessica Harwood
Douglas Wilkin, Ph.D.
EDITOR
Douglas Wilkin, Ph.D.
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Printed: January 15, 2015
CONTRIBUTORS
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Niamh Gray-Wilson
Jean Brainard, Ph.D.
Sarah Johnson
Jane Willan
Corliss Karasov
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C HAPTER
Chapter 1. Sex-linked Inheritance
1
Sex-linked Inheritance
• Define sex-linked trait.
• Explain the genetics of sex determination in humans.
• Explain sex-linked inheritance.
Male or female?
One of the exciting things about expecting a child is wondering if the baby will be a boy or a girl. There are many
superstitions about how one might influence or predict the outcome. But what really determines if a baby is male or
female? We now know that the gender of a baby is determined by a special pair of chromosomes known as the sex
chromosomes.
Sex-linked Inheritance
What determines if a baby is a male or female? Recall that you have 23 pairs of chromosomes—and one of those
pairs is the sex chromosomes. Everyone has two sex chromosomes. Your sex chromosomes can be X or Y. Females
have two X chromosomes (XX), while males have one X chromosome and one Y chromosome (XY).
If a baby inherits an X chromosome from the father and an X chromosome from the mother, what will be the child’s
sex? The baby will have two X chromosomes, so it will be female. If the father’s sperm carries the Y chromosome,
the child will be male. Notice that a mother can only pass on an X chromosome, so the sex of the baby is determined
by the father. The father has a 50 percent chance of passing on the Y or X chromosome, so there is a 50 percent
chance that a child will be male, and there is a 50 percent chance a child will be female.
One special pattern of inheritance that doesn’t fit Mendel’s rules is sex-linked inheritance, referring to the inheritance of traits that are located on genes on the sex chromosomes. Since males and females do not have the same
sex chromosomes, there will be differences between the sexes in how these sex-linked traits—traits linked to genes
located on the sex chromosomes—are expressed.
One example of a sex-linked trait is red-green colorblindness. People with this type of colorblindness cannot tell
the difference between red and green. They often see these colors as shades of brown ( Figure 1.1). Boys are much
more likely to be colorblind than girls ( Table 1.1). This is because colorblindness is a sex-linked, recessive trait.
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Boys only have one X chromosome, so if that chromosome carries the gene for colorblindness, they will be
colorblind. As girls have two X chromosomes, a girl can have one X chromosome with the colorblind gene and
one X chromosome with a normal gene for color vision. Since colorblindness is recessive, the dominant normal
gene will mask the recessive colorblind gene. Females with one colorblindness allele and one normal allele are
referred to as carriers. They carry the allele but do not express it.
How would a female become colorblind? She would have to inherit two genes for colorblindness, which is very
unlikely. Many sex-linked traits are inherited in a recessive manner.
FIGURE 1.1
A person with red-green colorblindness
would not be able to see the number.
TABLE 1.1: Cross Between a Female Carrier for Colorblindness and a Male with Normal Vision
Xc
Xc X
(carrier female)
Xc Y
(colorblind male)
X
Y
X
XX
(normal female)
XY
(normal male)
According to this Punnett square ( Table 1.1), the son of a woman who carries the colorblindness trait and a male
with normal vision has a 50% chance of being colorblind.
Summary
• Each individual has two sex chromosomes; females have two X chromosomes (XX), while males have one X
chromosome and one Y chromosome (XY).
• Sex-linked traits are located on genes on the sex chromosomes.
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Chapter 1. Sex-linked Inheritance
Explore More
Use the resources below to answer the questions that follow.
Explore More I
• Sex-linked Traits at http://www.youtube.com/watch?v=H1HaR47Dqfw (5:16)
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/282
1.
2.
3.
4.
What was unusual about the F2 generations in Morgan’s crosses?
According to Morgan, where is the gene for eye color located?
How did Morgan test his hypothesis on the location of the eye color gene?
What are three traits that humans have that are related to genes exclusive to the X-chromosome?
The "Morgan" referred to in the above clip is Thomas Hunt Morgan. You can find out more about him and his work
here: http://www.nature.com/scitable/topicpage/thomas-hunt-morgan-and-sex-linkage-452 .
Explore More II
• Inheritance of Sex-linked Traits at http://www.youtube.com/watch?v=IJqFk-28G08 (4:49)
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/57305
1.
2.
3.
4.
What are the three types of color blindness? How are they caused?
What is the "Law of Dominance"?
Can a woman have colorblindness if her father does not? Explain your answer fully.
A woman is color blind but her sister isn’t. What does that tell you about their parents
If you’re still puzzled by sex-linked traits you can go to this site for more practice solving problems. Make sure you
make good use of the "hints" on the site.
• Sex-linked genes at http://www.ksu.edu/biology/pob/genetics/xlinked.htm
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Review
1.
2.
3.
4.
What are the sex chromosomes of a male and a female?
Explain why the father determines the sex of the child.
What is sex-linked inheritance?
A son cannot inherit colorblindness from his father. Why not?
References
1. . A person with red-green colorblindness would not be able to see the number . Public Domain
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